26. A proposed analytical determination is the titration of a tin sample (analyte) with permanganate titrant (in acid soln) to determine the % Sn. The unbalanced redox reaction is:
Sn⁺²  +  MnO4^- -->   Sn⁺⁴  +  Mn⁺² (E0 for tin redox couple = 0.15 V, for manganese redox couple = 1.51 V)
a) balance the above reaction
b) Calculate the potential (vrs SHE) at the equivalence point for this titration. If you need a hydrogen-ion concentration value, assume [H+] = 0.10 M. ANS: 1.05 V

27. What is the % Fe in a 710.0 mg sample of iron ore that requires for complete titration 48.06 mL of a KMnO₄ titrant which has been standardized against Na₂C₂O₄ 268.0 mg of primary standard Na₂C₂O₄ requiring 40.00 mL of the KMnO₄ titrant to reach the equivalence point.

NOTE: the redox couples in each case are:
For standardization: MnO₄^-, Mn⁺² as C₂O₄^2-, CO₂
For the analysis: MnO₄^-, Mn⁺² as Fe⁺², Fe⁺³
(MW: Na₂C₂O₄ = 134.0, KMnO₄ = 158.0, Fe = 56.00) ANS: 37.91%
 

28. Given the following schematic representation of a galvanic cell:
Cu/Cu⁺² (0.02 M) // Fe⁺³ (0.01 M), Fe⁺²  (0.2 M)/Pt
a) Write the two half reactions and the overall cell reaction occurring in this cell.
b) Which half-cell is the anode? cathode? ANS: Cu - anode; Pt - cathode
c) Assign the appropriate polarity (+ or -) to each electrode.
d) Calculate the potential of the cell. ANS: 0.407 V
e) Calculate the equilibrium constant for the cell reaction. ANS: 4.86x10¹⁴
 

29. Draw the schematic representation and calculate the potential for the galvanic cell which results when the following two half-cells are connected together by a wire and a salt bridge: One half-cell consists of a gold electrode dipping into a solution which contains UO₂⁺² ion, 0.05 M, U⁺⁴  ion, 0.001 M and HCl 0.10 M. The second half-cell is a platinum electrode in a mixture of 0.10 M cerium (IV) and 0.05 M cerium (III) in a 1 M nitric acid solution. ANS: E = 1.362 V

30. In the standardization of a solution of KMnO₄ in 1 M HCl, it was found that 35.00 mL of this solution, when used as a titrant, was equivalent to 0.2500 gm of primary standard CaC₂O₄ (only 98.00% pure, however). In the titration, MnO₄ was turned into Mn⁺² while C₂O₄= (oxalate) became CO₂(g).
a) Write the balanced reaction.
b) Calculate the molarity of the permanganate (MnO₄) solution. (MW: CaC₂O₄ = 128.0; KMnO₄ = 158.0)
ANS: 0.02188 M
 

Clueless:

Part a) Straightforward redox reaction which needs to be balanced in acid conditions.

Part b) This is more extensive. You need to be able to set up an expression for the potential at the equivalence point for a titration which uses the two redox couples above in a titration reaction. Then you need to make substitutions to permit evaluation of the final value for potential.

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Hints:

Part a) If you need review, refer to Chem 111, P.S. 200

Part b) You need to write the Nernst expression for BOTH redox couples first. Then add them in such a way that (along with the Eo values for both redox couples) you have only one "log" expression in the resultant. This "log" expression will have present in it both redox couples along with [H3O+] if you find yourself having to have added H3O+ and H2O in the balancing of one (or both) of the original redox couples. This is not a simple problem and will take some study.

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Clueless:

This is a straightforward titration calculation, only for a redox titration instead of the more usual acid/base systems with which this course began. The data consists of 2 parts: a standardization of the titrant, followed by the determination of the analyte.

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Hints:

First determine the concentration of the permanganate from the primary standard data. This will require your balancing the redox reaction between KmnO4 and Na2C2O4. Once you know the concentration of the MnO4- solution, use it as the titrant in the second redox reaction, between MnO4- and Fe+2.

 

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Clueless:

An exercise in interpreting a schematic representation of a galvanic cell and writing down the data which this representation is telling you.

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Hints:

Since you are told you have a galvanic cell, you can write down the two half-reactions which are actually occurring. This in turn will set up the corresponding Nernst expressions for each redox couple, by which you will be able to calculate parts d) and e)

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Clueless:

Here you need to use the information given you to first calculate the reduction potential for each half cell. Only then will you be able to set up the galvanic cell which results when these two half cells are connected electrically but not physically. This allows you to write the schematic representation.

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Hints:

Calculate the reduction potentials for each redox couple. You should now be in position to set up the galvanic cell and write its schematic representation. Once you have this you know how to interpret a schematic representation and to calculate Ecell.

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Clueless:

A standardization calculation for a redox titration.

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Hints:

The only "wrinkle" here is that the "primary standard" is only 98.00% pure. You need to take that fact into account when coming up with just how much actual primary standard was present in the amount you weighed out.

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